The density of a substance, its mass divided by its volume, can be an identification parameter for pure substances, a composition descriptor for mixtures of two pure substances, an indicator of adulterations in preparations of otherwise known composition and a detector of voids in an apparently solid object.
Density must be accurately determined to provide the above information. The mass of a solid substance is easily and accurately determined with modern analytical balances, and volume is also easily and accurately established for solid substances of uniform geometric shape such as spheres and cubes. However, volume of irregularly-shaped solids such as minerals, commodities and many other substances of commercial significance, is not easily calculated.
Commercially available gas comparison pycnometers typically measure volumes of 50 cm.sup.3 to a precision of .+-.0.1 cm.sup.3. Greater precision is sorely needed if the measured density is to function as an identification parameter, a composition descriptor, an indicator of adulterations or a detector of voids. As is well known, such pycnometers operate by pressurizing gas around a sample in a chamber, measuring the pressure, allowing expansion of the gas into an additional chamber, and measuring the pressure in the combined volume. The result is compared to calibration data to determine the volume of the unknown sample.
Current gas comparison pycnometers suffer from many deficiencies which limit the precision of the volume determination. First, the devices are not stable against ambient temperature variations. Any change in room temperature can influence the pressure measured in either the sample chamber or the expansion chamber.
Second, when an expansion is made from the sample chamber into the expansion chamber, an internal gas energy change transpires. Its influence cannot be accounted for except by insuring that the pressure change encompasses identical limits.
Third, closure devices for the sample chamber do not adequately seal and reproducibly define the chamber to insure a constant volume when the test are run. An inadequate closure of the sample chamber greatly decreases the accuracy of the volume determination.
Fourth, problems arise when the sample is placed into the pycnometer. The sample may be at a temperature different from the pycnometer. Also, unwanted moisture and vapors enter the system when the sample is introduced into the pycnometer. The sample may itself contain large quantities of vapors which will cause the pressures to vary from the ideal values. These foreign vapors must be removed before a precise volume measurement can be achieved. Heretofore, unwanted moisture and vapors have been removed from the pycnometer by attempting to completely evacuate the system with a vacuum pump. Other devices remove the unwanted moisture and vapors by a prolonged flushing of the system with a steady, slow flow of helium. Other devices try to purge the system by manually alternatively increasing and decreasing the gas pressure in the sample chamber.
The above deficiencies can greatly affect the accuracy of the volume calculated by the pycnometer. However, no prior art pycnometers have a way to check the precision of its volume determination. Thus, there has been a need in the art for an apparatus exhibiting greater precision when determining the density of a solid substance.